GRHD: Implement TOV test in 2d axisymmetry using the cartoon formulae

The examples/grhd_tov/dg_cartoon.toml example can run for a good while without immediately crashing. This means that the bdry conditions (reflection symmetry across x = 0 and z = 0 axes) aren't so wrong and the surface treatment is doing something.

However, ... I realized that I have been running with smaller time steps than other schemes do. That is, mine scales as ~ N^{-3}, but it should more be like ~ N^{-2}. This explains why I could run with comparatively large CFL numbers some of the 1d tests. This should be investigated in a separate PR. Changing to ~N^{-2} here makes the cartoon test crash shortly after start, because of an issue in the cons2prim recovery. So perhaps we can't get away without any regularization scheme and I have to implement AV too.